Volumetric molecular imaging to determine the chemical composition of an object has become critically important in a number of application areas, such as homeland defense, port inspection, and airport security. Promising techniques for performing such analysis include interrogation of the object with x-ray radiation, which has been relied upon for years to provide insight into the structure of objects in the areas of baggage inspection, medical imaging, and semiconductor crystal analysis, among others.
X-rays can interact with materials in a number of ways, including photoelectric absorption, Compton scatter, coherent (Bragg) scatter and fluorescence. Historically, conventional x-ray inspection systems have been used only to measure variations in the manner in which the x-rays propagate through an object (e.g., absorption). Unfortunately, this does not provide information about molecular or atomic identity.
The chemical composition information about molecular or atomic identity information can be obtained by analyzing the scatter behavior of x-rays directed at an object, however. There exists a well-known relationship between a material's molecular structure and the angle at which a given x-ray will scatter from it. As a result, the molecular structure a material gives rise to unique scattering behavior, such that each material has a scatter “signature” that can be used to identify it. In recent years, a number of techniques have been developed to exploit this phenomenon.
Coherent scatter computed tomography (CSCT) is one such approach for providing molecular imaging capability. In a typical CSCT system, a poly-energetic x-ray beam, typically pencil-shaped, is directed at an object to give rise to low-angle coherent-scatter x-ray diffraction. The scatter radiation is then detected and used to identify the molecular structure. Examples of CSCT systems are found in, for example, U.S. Pat. No. 7,580,499, “Energy-dispersive coherent scatter computed tomography, Applied Physics Letters, 88(24):243506 (2006), “Angular-dependent coherent scatter measured with a diagnostic x-ray image intensifier-based imaging system,” Medical Physics, 23(5):723-733 (1996), and “x-ray diffraction computed tomography,” Medical Physics, 14(4):515-525, (1987).
Another approach to molecular imaging is energy dispersive x-ray diffraction tomography (EXDT), examples of which are disclosed in U.S. Pat. No. 7,835,495 and “Energy-dispersive x-ray diffraction tomography,” Physics in Medicine and Biology, 35(1):33 (1990). In this approach, a fan-shaped x-ray beam and a plurality of pencil-shaped x-ray beams are directed at an object and the scatter radiation is received at a plurality of detectors that includes a transmission detector array and a plurality of scatter detectors. Information about the molecular composition of the object is generated from the output signals of all of the detectors.
Other prior-art approaches include kinetic-depth-effect x-ray diffraction (KDEXRD), which is disclosed in “Combined x-ray diffraction and kinetic depth effect imaging,” Optics Express, 19(7):6406-6413, (2011), and coded-aperture x-ray scatter imaging (CAXSI), disclosed in “Pencil beam coded aperture x-ray scatter imaging,” Optics Express, 20(15):16310-16320 (2012).
Each of these approaches, however, have drawbacks that limit their practical utility. For example, poor utilization of the incident photons gives rise to a weak coherent scatter signal. In addition, therefore, many of these approaches result in the capture of the scatter from only a small fraction of the radiation directed at the scanned object and are, therefore, highly inefficient. As a result, in order to produce an output signal having sufficiently high SNR over a short time, they require either x-ray sources capable of high power to increase the available radiation at the detector, or long exposure times. In either case, this exposes the scanned object to excessive amounts of x-ray radiation, which can be undesirable in many applications. Further, prior-art systems rely on the use of pencil or fan beams to sequentially interrogate small sections (e.g., a single voxel or planar slice) at a time, which can be very time consuming. The need to develop the tomographic model of the object one portion at time can lead to an undesirable space-time spectral trade-off.
As a result, it is difficult, at best, to employ such approaches in a real-time molecular imaging system. There remains a need, therefore, for an improved imaging system that noninvasively ascertains the structural and molecular composition of three-dimensional objects at high speed and with relatively lower cost and complexity.